1. Field of the Invention
The present invention relates to a door phone system in which a door phone image is displayed on a portable wireless monitor slave unit using a wireless communication.
2. Description of the Related Art
A conventional TV door phone system is configured by a camera equipped front door slave unit and a monitor equipped indoor master unit wired connected to a corresponding front door slave unit. However, in order to easily respond to a visitor even when a respondent is not around the indoor master unit, it has been recently proposed that an image is displayed on a portable display device (wireless monitor slave unit) using a wireless communication and that a voice answering to the corresponding front door slave unit is allowed in the wireless monitor slave unit (see Patent Document 1). In the TV door phone system described in Patent Document 1, when a wireless monitor slave unit does not exist in a range of a wireless communication of the indoor master unit, a public switched telephone line is used to call the wireless monitor slave unit, and with only one wireless monitor slave unit, a one-to-one communication is made between the indoor master unit or the wireless base station and the wireless monitor slave unit. When a visitor manipulates a call switch of the front door slave unit, the indoor master unit having a wireless communication function checks whether there is a wireless monitor slave unit in a range of the wireless communication, and if so, transmits image data from the indoor master unit to the wireless monitor slave unit. Further, the visitor image is displayed on the wireless monitor slave unit and at the same time, when there is an answer from the wireless monitor slave unit, a communication path is established, so that a telephone call between the front door slave unit and the wireless monitor slave unit is enabled. In addition, a TV door phone system has been proposed to which a plurality of wireless monitor slave units can be connected (see Patent Document 2).
[Patent Document 1] JP-A-1997-107409
[Patent Document 2] JP-A-2000-92235
In the TV door phone system as described above latent Document 2), when a visitor manipulates a call switch of the front door slave unit, an indoor master unit having a wireless communication function calls a plurality of wireless monitor slave units through a control channel, and a communication channel is established between the indoor master unit and an answered wireless monitor slave unit, and a telephone call and image data transmission is performed with the front door slave unit through the communication channel. Therefore, in the door phone system as described in Patent Document 2, there is a drawback in that the visitor image can be checked before answering. As such, the conventional wireless monitor appended TV door phone system makes a call after displaying images of the front door slave unit on one wireless monitor slave unit, or calls a plurality of wireless monitor slave units, but it cannot display images of the front door slave unit on the plurality of wireless monitor slave unit before answering.
In addition, generally, an apparatus that communicates image information or voice information wirelessly establishes a one-to-one two-way wireless link at a transmission side and a reception side, to thus make a communication. For this reason, in case of sending image informing to a plurality of cordless slave units at the same time, it is necessary to have a plurality of wireless units at the transmission side. In addition, when a wireless device is used that makes a multiplexing communication such as TDMA, a plurality of communications can be made at the same time without having a plurality of wireless devices such as transmitters, receivers, and antennas. Further, it is possible to transmit image information to the plurality of cordless slave units at the same time. However, in case of the TDMA scheme, there is an upper limit to the number of slots accepted in one frame, so that there is a limit to the number of simultaneous transmissions. In addition, in case that simply broadcasted and transmitted image information is simply displayed on the wireless monitor slave unit, when the wireless monitor slave unit cannot receive a call signal or image information transmitted from the monitor appended indoor master unit due to a wireless communication error, there is a problem in that a calling sound does not ring or an image is susceptible to be distorted. In addition, in case of a two-way communication, it is possible to detect a wireless interference with other devices based on a poor receiving state, and to avoid interference by changing a communication frequency. However, when image information is transmitted in the broadcasting transmission, it is not possible to detect a receiving error state at the transmission side (broadcasting source), so that there is a problem in that it is not possible to avoid interference.